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Old 18th September 2012, 11:20 PM   #2821
agdr is offline agdr  United States
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Quote:
Originally Posted by Amarphael View Post
I have about -5.8V on the pot, U1 ( pin1 -5.81V, pin8 -6.6V), Also on the gain switch and gain resistors, but the resitors near the input (r3,7,14,20) are fine. Output is fine and U3+U4 also fine.
See if you can take DC voltage readings on all the pins of U1 (the NJM2068) to ground (metal shell of the gain switch). Pins 3 and 5 especially should read 0Vdc. I suspect your source device may have DC on the input signal feeding the O2. Since the O2 doesn't have any input coupling caps any source DC will whiz right through and stop after the pot with C13 and C14.
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Old 19th September 2012, 02:58 AM   #2822
agdr is offline agdr  United States
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Amarphael:

After re-reading what you wrote the problem may not be source DC. Sounds like you measured some low voltages on the input resistors, and your other O2 is working presumably with the same sound source. BTW I should have said "near 0Vdc" on pins 3 and 5 since there will be some tiny voltage equal to the njm2068 input bias current times 10K or so.

But... a couple other ways to get the -5.8Vdc and -6.6Vdc on the pot are the NJM2068 being bad, or something gone wrong in the feedback loops on both sections of the chip to cause very high stage gain, so the NJM2068 is acting more like a comparator than an amplifier. That -5.8Vdc and -6.6Vdc are getting kind of close to the +/- 7Vdc clipping level that happens if that stage is all the way "on" in either power supply rail direction, which is what a comparator would do.

So then another thing to check here is that you didn't accidentally get the U1 (NJM2068) and U2 (NJM2903D) chips swapped. They are pin compatible and would actually run swapped, but the results would be something like you are getting. The power management circuit would probably work pretty normally with the NJM2068 in for U2, but not so for the gain stage with the NJM2903D in for U1. The other thing to try is after taking those U1 voltage readings above, power the O2 down and remove U1, then power it back up and make the same voltage readings on all the U1 IC socket pins with that chip out. Probably a good idea to leave your headphone unplugged to avoid any damage until the problem with the pot voltages is resolved.

Last edited by agdr; 19th September 2012 at 03:07 AM.
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Old 19th September 2012, 11:50 PM   #2823
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Quote:
Originally Posted by agdr View Post
The other thing to try is after taking those U1 voltage readings above, power the O2 down and remove U1, then power it back up and make the same voltage readings on all the U1 IC socket pins with that chip out. Probably a good idea to leave your headphone unplugged to avoid any damage until the problem with the pot voltages is resolved.
Yeah, I did measured all of them but by the time i submitted the post i got logged off and it MIA and i've remebred only those two... Anyway, reading are as follows w/chip and w/o:
pin1 w/ -5.81V, w/o 0V;
pin2 w/ -2.3V, w/o 0V;
pin3 w/ 0V, w/o 0V;
pin4 w/ -11.7V, w/o -11.7V;
pin5 w/ 0V, w/o 0V;
pin6 w/ -2.3V, w/o 0V;
pin7 w/ -5.81V, w/o 0V;
pin8 w/-6.6V, w/o -0.004V.

the NJ2068 it self is fine because i swapped the other from the working unit but the readings are the same. Also, no i didn't reverse u1-u2, it's 2068 and 2903 respectively.

So how do i check those feedback loops?

Last edited by Amarphael; 19th September 2012 at 11:52 PM.
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Old 20th September 2012, 02:57 AM   #2824
agdr is offline agdr  United States
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Quote:
Originally Posted by Amarphael View Post
Anyway, reading are as follows w/chip and w/o:
The problem is pin 8 of U1, the positive voltage rail. That should be somewhere around the opposite of the negative power supply rail on pin 4, +11.7Vdc or so. With that positive rail missing the NJM2068 is just doing the best it can with all the other voltages.

Most likely your Q1 mosfet isn't turning on. See what voltages you are getting on pins 4 and 8 of U2, the NJM2903, during a time when you are getting that -6.6Vdc for pin 8 of U1, with U1 installed in the socket. Also see what you are reading on pins 1 and 7 of U2.

Last edited by agdr; 20th September 2012 at 03:06 AM.
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Old 20th September 2012, 11:37 AM   #2825
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Did anyone try to use the power supply from the US Robotics Courier modem ? It's rated 20VAC, but measure closer to 23 V. Will it be a big issue to use it ?

Best Regards,

Davide
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Old 20th September 2012, 02:09 PM   #2826
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23vAC is ~32V peak when rectified. The recommended limit for the 7812 voltage regulators from most manufacturers is 35V. Fluctuations in the mains supply may cause this voltage to be approached or even exceeded. In the UK mains voltage is specified to be +/-10%.

Of course you could have been measuring on a high day.

Although the voltage will probably be slightly less when the transformer is loaded, the caps in the power rectifier section are only rated 35V. Again, this is slightly more than they will be exposed to, but for longevity they are usually only exposed to 2/3 of their rated voltage.

You will probably get away with using this transformer, particularly in the short term, but there will be 20V dropped across the regulators, causing them to run hot. The increased temperature within the case will shorten the life of the caps.

If the amplifier fails after a comparatively short life, you will have little reason to complain. The voltage from the transformer is certainly in the range that would be considered risky. The documentation recommends transformers in the range 13.5-20v no load.
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Old 20th September 2012, 02:22 PM   #2827
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Thanks, I actually use it only to recharge the batteries, soforshort period. I will give it a try. Worst case, I will have to replace two capa.

Thanks,

Davide
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Old 20th September 2012, 04:41 PM   #2828
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Quote:
Originally Posted by agdr View Post
Most likely your Q1 mosfet isn't turning on. See what voltages you are getting on pins 4 and 8 of U2, the NJM2903, during a time when you are getting that -6.6Vdc for pin 8 of U1, with U1 installed in the socket. Also see what you are reading on pins 1 and 7 of U2.
Yeah that's what i was thinking but the 2903 readings were exactly righ to begin with:

pin1 -11.7V
pin4 -11.7V
pin7 9.5V
pin8 11.7V

Other four are also ok.

Q1 measures (from the back of the board):
left: -11.7V
middle: -11.7V
right: -11.2V

Q2 reads the same first two, 9.3V on the right.

But i do have the 'imbalance' where D4 (unbended) is 0.4V lower than D3 (banded) when powered on, but are fine (less tan 0.1V) when powered off, ACV connected. Which should be fine according to the TS as it refers to the latter situation (powered off).

Last edited by Amarphael; 20th September 2012 at 04:53 PM.
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Old 20th September 2012, 08:45 PM   #2829
agdr is offline agdr  United States
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Quote:
Originally Posted by Amarphael View Post
Yeah that's what i was thinking but the 2903 readings were exactly righ to begin with:

pin1 -11.7V
pin4 -11.7V
pin7 9.5V
pin8 11.7V

Other four are also ok.

Q1 measures (from the back of the board):
left: -11.7V
middle: -11.7V
right: -11.2V

Q2 reads the same first two, 9.3V on the right.
Interesting! You are narrowing it down. That leftmost pin of Q1 with the -11.7Vdc on it (looking at things from the back of the PCB with the jacks at the top) is the +11.7Vdc input from the power switch and is hooked directly to pin 8 of U2, where you measured the +11.7Vdc, by a tiny trace on the top of the PCB that runs under a bunch of components.

On the back you probably had your meter lead on the Q1 lead sticking out. If you can get your meter probe on the actual copper pad around that leftmost Q1 pin you may get the +11.7Vdc. May be a bad solder joint there or a bridge. Try reheating all 3 Q1 pins with the soldering iron for about 2 full seconds each. Then also take a look between them with a magnifying glass for bridges and good solder coverage all around the pins. Can be really hard to see bridges between those mosfet pins.

Last edited by agdr; 20th September 2012 at 09:14 PM.
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Old 28th September 2012, 10:19 PM   #2830
tER299 is offline tER299  Netherlands
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Default Problem with Objective 2 output

Hello,

I'm new to diyAudio and new to diy amplifiers too. I was wondering if anyone could help me with a problem I have with my newly built O2 amp.

The sound of my amp is great, but from time to time, if either the input or output cable moves, the sound in of my left earphone disappears. I can get it back when I jiggle the cables, but from time to time (whenever the one of the cables move to much) the sound disappears again. I have no multimeter at hand, but my best guess is that the 3.5 input and output are broken, but it seems as if the problem doesn't occur as often when my batteries are almost empty. Could someone confirm this, or give suggestions on what the problem might be?

Any help will be greatly appreciated!!
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